The present invention relates to a device with an osmotic drive for administering an injectable product, in particular an infusion pump for administering insulin or the like over a long period of time.
In many medical or therapeutic applications, it is necessary or advantageous to administer a medicinal or therapeutic substance at a constant or variably adjustable continuous flow volume, rather than administering a large one-off dose of substance. For this purpose, it is common practice in in-patient treatment to mount infusion bottles containing a liquid substance above an injection point so that the liquid substance is delivered to the injection point via a hose connection by force of gravity. However, such infusion bottles are not suitable for use with an administering device in mobile applications or for administering relatively small doses over a long period.
Infusion pumps with a separate drive have therefore been developed, by means of which a product in a product container can be continuously delivered under pressure, enabling the product to be continuously dispensed from the container. U.S. Pat. No. 4,838,862, for example, discloses a portable infusion device with an osmotic drive, comprising a flat housing with several chambers lying one above the other and a laterally disposed outlet for the product. The uppermost chamber constitutes a solvent chamber, which is filled with water and is separated from a middle solution chamber filled with a salt solution by means of a fixed support plate, a porous layer disposed underneath and a semi-permeable membrane disposed under it. The fixed support plate has an orifice, which is sealed by means of a foil seal. A bottom product chamber is separated from the middle solution chamber by an elastic diaphragm. In the product chamber, the outlet for the product is provided in the form of a lateral orifice in the housing. The infusion device may be placed at a point on the body of a user by means of the base surface of the housing, for example, and carried in this manner. A push-button projects upwards out of the housing, disposed above the foil seal and has a needle pointing inwards into the water chamber in the direction of the foil seal. The infusion device is activated by pushing the push-button into the housing so that the needle pierces the foil seal and forces water out of the top water chamber through the orifice in the fixed support plate into the porous layer via the semi-permeable membrane. An osmotically induced pressure is generated in the middle solution chamber via the semi-permeable membrane by the water, which acts on the elastic diaphragm and hence on the product chamber. As a result of the pressure on the product chamber, the product is administered through the outlet at a continuous flow volume.
The elastic diaphragm of this known infusion device is disposed above the product chamber across a large surface area in order to create sufficient pressure in the chamber. Although the elastic diaphragm is able to camber inwards into the product chamber, this does not enable the product to be fully dispensed from the product chamber. Relative to the total surface area of the solvent chamber, the push-button overlaps with a small surface area. When the button is depressed, little pressure is generated in the solvent chamber, which mans that the solvent is distributed across the total surface of the semi-permeable membrane solely by the capillary action of the porous layer.